Here is what ancestry.com has on a William H. Barnes>>

Maryland Soldiers in the Civil War, Vol. 2

Name: Barnes, William H.
Rank: Private.
Enlistment/Muster In: Sept. 26, 1863
Discharge/Muster Out: Nov. 16, 1866

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Civil War Pension Index: General Index to Pension Files, 1861-1934
Name: William H. Barnes
State Filed: Maryland

Note: There are four Cival War pensions given to William H Barnes of Maryland. They are dated from 1890 to 1927.

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So if in fact Mr. Barnes did not die in 1866 but rather died in 1936, then this maybe him in these Census records from Saint Mary's County>>

1930>

1930 United States Federal Census
Name: William Barnes
Home in 1930: District 8, Saint Mary's, Maryland
Age: 90
Estimated Birth Year: abt 1840
Birthplace: Maryland
Relation to Head of House: Head
Race: Black

Household Members:
William Barnes 90
Archie Barnes 40
Agnes Barnes 35
Earnest Barnes 38
Bernice Barnes 13
Joseph Mitchel 55

In 1920>

1920 United States Federal Census

Name: William H Barnes
Home in 1920: Bay, Saint Mary's, Maryland
Age: 75 years
Estimated Birth Year: abt 1845
Birthplace: Maryland
Relation to Head of House: Head
Spouse's name: Minnie
Father's Birth Place: Maryland
Mother's Birth Place: Maryland
Marital Status: Married
Race: Black
Sex: Male

Household Members:
William H Barnes 75
Minnie Barnes 60
Stephan F Barnes 30

In 1910>>

1910 United States Federal Census

Name: William Barnes
Age in 1910: 68
Estimated Birth Year: abt 1842
Birthplace: Maryland
Relation to Head of House: Head
Father's Birth Place: Maryland
Mother's Birth Place: Maryland
Spouse's name: Mymy
Home in 1910: Election District 8, Saint Mary's, Maryland
Marital Status: Married
Race: Black
Gender: Male

Household Members:
William Barnes 68
Mymy Barnes 54
Eliza Barnes 31
Stephin F Barnes 22
James T Barnes 16
Mannie A Barnes 12
Florence Barnes 11

In 1900>

1900 United States Federal Census

Name: William E Barnes
Home in 1900: Centerville, Saint Mary's, Maryland
Age: 60
Estimated Birth Year: abt 1840
Birthplace: Maryland
Relationship to head-of-house: Head
Spouse's name: Munsey
Race: Black

Household Members: Name Age
William Barnes 60
Munsey Barnes 48
Eliza Barnes 19
Stevan F Barnes 10
Bristol Barnes 9
James T Barnes 6
Mannie A Barnes 2
Elenora Barnes 3.12
Thomas Barnes 1

In 1880>>

1880 United States Federal Census

Name: William Barnes
Home in 1880: Hillville, Saint Mary's, Maryland
Age: 32
Estimated Birth Year: abt 1848
Birthplace: MD
Relation to head-of-household: Head
Spouse's name: Myrna
Father's birthplace: MD.
Mother's birthplace: MD.
Occupation: Laborer
Marital Status: Married
Race: Black
Gender: Male

Household Members:
William Barnes 32
Myrna Barnes 25
Sarah E. Barnes 1
Abram Barnes 70

Tony